Workbench Power supply

Discussion in 'The Projects Forum' started by Regected, Apr 30, 2010.

  1. Regected

    Thread Starter Active Member

    May 24, 2009
    I've been tooling around with building a desktop power supply for some time now. It started out as just a simple modified AT power supply, which gave me +12, +5, -5 and -12 VDC. I added a piece of plexi with a selector switch, volt and amp meters, a 30 amp circuit breaker and both output lugs and 5 way binding posts. This worked well enough till I needed more than the available 5 amps.

    The next iteration was the addition of a second transformer and another selector switch. The transformer is a microwave oven transformer from McDonalds, with the the secondary winding removed and rewound with 12 gauge sold wire (a single strand from romex). I get 24 VAC unloaded with an optional center tap(not used at this time). I then added a 40 amp bridge rectifier and a large cap to give a simple, high current DC power supply. This works well enough. It holds 30 volts with up to a 8 amp load, then starts dropping from there and ends up at 24v at 30 amps. Don't think that's too bad for having next to no idea what I'm doing. It works well enough, but time has come to make a a more reliable voltage source for high current devices. With out any regulation, I've fried a handful of projects, so this got pushed up on my list of priorities.

    The AT power supply side is going to stay as it is, but I want to add voltage regulation and *maybe* current regulation. What I was thinking about building is something like a buck dc-dc power supply. Using a LM317 as an adjustable reference voltage, I want to use an opamp to switch a set of mosfets. On the output, I am planning on using a capacitor directly fed from the mosfets and an inductor to filter the output. Anyone have a diagram for something like this? I've been poking around here and on google, but have had no luck. Also, any ideas for adding current regulation?
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    What is your curent 'CURRENT' goal?

    Do you want 30A? Do you NEED 30A?
  3. SgtWookie


    Jul 17, 2007
    Romex should not be used for winding transformers. It won't take the heat. You should use magnet wire of the appropriate gauge.
  4. Regected

    Thread Starter Active Member

    May 24, 2009
    My goal is 30 amps. I don't have anything at this time that requires that much current, but I'm "that guy" that is always building off the wall stuff.

    Thanks for the heads up Wookie. I just used what I had on hand. Being an EE student, funds are very tight. If anything happens with the current windings, I'll swap in some more appropriate wire. The highest load I've had on ti so far has been 15 amps, and it took it like a champ.
  5. Regected

    Thread Starter Active Member

    May 24, 2009
    After doing more digging, I've found a very basic diagram for similar to what I was thinking. It adds an oscillator and moves the capacitor to after the inductor. The voltage reference is fixed and the sampling reference is variable. I guess this would be a good place to start, and modify it to fit my needs. I don't believe I need the oscillator. It could increase the switching speed to reduce the output ripple, but add a level of complexity that would be undesirable for me. I also don't think the sampling circuit will work for me. I need to be able to get down to around 1v, which would put the reference voltage that low; that low of a reference voltage does not sound like a good idea. Adding a selectable voltage divider on the reference voltage output could overcome this limitation.

    Don't mind my rambling. I'm just typing as I think. A sane man would hit delete rather than submit.
  6. someonesdad

    Senior Member

    Jul 7, 2009
    Regected, I feel quite uncomfortable seeing the open conductors with line voltage on them. Please tell me that it normally goes in a box so that things are insulated well (I may have missed it if you said it).

    I do remember what it's like being a student and not having much in the way of resources to help with building things, so your methods are understandable. One of the things you might do is look for some nearby alumni that look like they might have technical businesses. You might lay your situation out to them and find that you have some sources for parts. If I was so approached by a nearby enterprising (yet polite and creative) student, I'd bend over backwards to get them stuff to help them along.

    More importantly, such contacts might result in some part time work or consulting, helping to generate a little revenue.
  7. Regected

    Thread Starter Active Member

    May 24, 2009
    Anything with mains voltage is insulated with shrink wrap or some other form of insulation. Once this is finalized, I'm going to finish the missing three sides of the enclosure. I just don't want to have to build it several times through the course of developing it.

    That a good idea about the alumni. Unfortunately, I'm getting my EE from a tech school rather than a traditional university, so finding an alumni that has actually gone on and done something worth while in the field is a big order. ON the plus side, I feel like a man among mice in school. I have been top of my class since day 1 and I even push the professors to dive deeper into the curriculum to keep me supplied with material to learn.

    On the up side of being low on funds, I've gotten very good at dumpster diving. It has even netted me a working 68 inch projection screen TV.